The giant black hole in the middle of our galaxy stays pretty quiet most of the time, flaring up only occasionally. But it is due for a burst of activity any day now, as a large cloud of gas and dust continues to spiral toward the heart of the Milky Way.

With this sudden influx of material, the normally tranquil black hole — named Sagittarius A* (pronounced "A star") and as massive as 4 million suns — will roar to life, unleashing a fiery discharge of matter and radiation.

The cloud, dubbed G2, is expected to make its closest approach this year, although it could take decades for the black hole to finish digesting its ethereal prey. Sagittarius A* doesn't dine often, but when it does, it is (like other black holes) a messy eater, drawing in far more material than it can swallow. The vast amount of gas and dust sucked in by the black hole's intense gravitational pull creates an enormous traffic jam that prevents most of this stuff from ever making it into the black hole. Instead, the material keeps piling up.

As the pressure mounts, atoms and smaller particles grind against each other, heating to temperatures of billions of degrees. With no way in, the now-energized stuff ricochets back into space at nearly the speed of light, forming extended, luminous jets aligned along the black hole’s powerful magnetic fields. At least that’s how theorists think it works; they’re still in the dark about many details concerning these jets.

Nor can anyone predict exactly how the encounter between Sagittarius A* and G2 will play out. “We don’t know what’s going to happen, but we do know there could be some amazing fireworks in the galactic center,” says Shep Doeleman, an astronomer at MIT’s Haystack Observatory and the Harvard-Smithsonian Center for Astrophysics. “This may be a once-in-a-lifetime opportunity for observers because Sagittarius A* is the Goldilocks black hole, being large enough and close enough to resolve with Earth-based telescopes.” Consequently, astronomers all over the world have their sights trained on the galaxy’s inner core as the cloud makes its terminal plunge.

Although numerous devices will be able to measure different aspects of the ensuing feeding frenzy, only one telescope has a chance of obtaining actual pictures of this explosive event and observing it in real time. Doeleman leads the team assembling this singular instrument, called the Event Horizon Telescope (EHT), which could show, for the first time ever, what actually happens when a sizable blob of matter falls into a mammoth black hole. “Will the cloud plunge straight into the black hole, or will some of it wrap around the side and spin off?” he asks. No one knows for sure because it has been impossible to see such a thing — until now.

Astronomers can't wait for the gas cloud known as G2 to reach our galaxy's central supermassive black hole, as shown in this simulation.

ESO/MPE/M.Schartmann

Eyes on the Prize

The EHT is so named because it will provide as close a look at a black hole as we can muster at this moment, carrying us virtually to the edge of the invisible boundary surrounding it — a spherical shell known as the event horizon. A black hole is an object that has collapsed upon itself so violently that it’s almost infinitely dense at its center. Its gravitational tug is so fierce that once matter or light gets close enough and crosses the event horizon, there’s essentially no turning back. It’s trapped inside the universe’s ultimate roach motel.

Unlike a traditional telescope with a single large mirror or antenna, the EHT consists of a coordinated network of radio telescopes spread across the world. Telescopes in Arizona, California and Hawaii are already connected in this way, and Doeleman soon hopes to bring other existing telescopes, at a half-dozen or so spots around the globe, into the EHT fold. By linking up widely separated antennas, freezing the light they capture and creating a composite picture, the result is effectively “a mirror as big as the Earth,” he says.

This general approach of blending the input from dispersed telescopes, called interferometry, offers the potential for vastly enhanced angular resolution — the ability to identify distinct features of an object that are spaced close together on the sky. The bigger a telescope’s aperture, or “eye,” the smaller the features it can detect. The EHT, which is intended to span our planet, takes this notion to the extreme: Its vision, which still has room for improvement, is already 2,000 times sharper than that of the Hubble Space Telescope.

The main technical challenge with the EHT is getting all the telescopes in the network to function as a single, well-oiled machine. Doeleman, who thrives on technical challenges, is well suited for this task, which almost feels like a hobby to him. An incorrigible fixer, he regularly repairs the wiring in his home in Malden, a northern suburb of Boston.

Doeleman, a trim and youthful-looking 47-year-old, got started on this track as a researcher and instrument repairman in Antarctica shortly after graduating from Reed College in 1986. “Doing interesting science under difficult circumstances made a huge impression on me,” he says. The night sky in Antarctica, “where the stars are literally blazing,” also made an impression, as he ultimately decided to pursue astronomy in graduate school at MIT. Once there, he did research at Haystack, studying the “magical technique” that underlies the EHT — using multiple observatories to generate a much sharper image. He continued this work after earning his Ph.D. in 1995, taking a job back at Haystack in 1998.

After a few years, Doeleman began to wonder whether the interferometry techniques he’d been experimenting with could finally reveal what was happening in the core of the Milky Way — all the way down to the boundary of the prodigious black hole lurking therein. In 2006 he and his colleagues took a look. Relying on telescopes in Arizona and Hawaii (which would eventually be part of the EHT), they tried to get observations of Sagittarius A* at higher angular resolutions than had been possible before, but failed to detect the signals they expected to see in the immediate vicinity of the black hole.

Inside the Event Horizon Telescope

One way to see how different the EHT is from your average telescope is to visit Doeleman’s prime stomping grounds: the Haystack Observatory, about 30 miles northwest of Boston. Haystack has a newly refurbished 37-meter-diameter (120-foot) radio telescope that may soon be added to the global array. But the observatory is home to an even more critical piece of equipment — the EHT’s “lens.”

Whereas a conventional telescope commonly has a mirror or lens shaped like a parabola to concentrate the light it gathers onto a point, the lens at Haystack is a 6-foot-high stack of computers. This device, or correlator, “is a special kind of supercomputer that is very good at doing just one thing,” Doeleman says. “We take the light recorded at different stations around the world, compare it and combine it, replicating with electronics what a mirror does with geometry.”

Each EHT station also houses a special kind of atomic clock, called a hydrogen maser. Just as the quartz crystal in a Swiss watch keeps time by vibrating at a steady frequency (about 32,000 cycles per second), the maser coaxes a supply of specially selected hydrogen atoms to emit radio waves at an unwavering 1.42 billion cycles per second. The maser’s higher frequency gives it far greater accuracy.

That careful timekeeping enables scientists at Haystack to correlate observations made at different spots on Earth’s surface, taking into account the curvature of the Earth and the location of each telescope to within a centimeter. “This clock loses only one second every 100 million years,” says Doeleman. “That’s how stable it is, and that’s the level you need.”

Shep Doeleman, at the Large Millimeter Telescope in Mexico, has traveled extensively to assemble the planet-spanning Event Horizon Telescope.

Igor Jimenez

After months of investigating, they found out what was wrong, uncovering a defect in a radio receiver in Hawaii. “A lot of science,” says Doeleman, “is picking yourself up and dusting yourself off and trying again.” That’s exactly what they did in 2007, using the same telescopes in Arizona and Hawaii, with a third telescope from California thrown in to round out the initial trio of EHT stalwarts.

Reviewing the data the second time around, Doeleman started to get “a tingly feeling.” He realized that the radio emissions from Sagittarius A* — emanating from the white-hot material swirling around the black hole like water circling a bathtub drain — were coming from a region that was significantly smaller than expected. For practical reasons this was extremely fortuitous, as most astronomers had assumed the black hole would be too big to get a good look at, he explains, “like putting your face right next to the wall and trying to describe it.”

Doeleman and his cohorts not only saw the wall; they came away with some of the highest-resolution observations ever achieved in astronomy. It was comparable to spotting a baseball on the surface of the moon. This was the first successful test of the EHT concept, says Doeleman, “the ‘aha!’ moment when we knew we were really onto something. We knew that we finally had access to a region of space-time” — just outside the black hole’s event horizon —“that had, until now, eluded astronomers.”

But the true objective of the EHT is yet to come. So far, Doeleman and company have only been able to establish that radio emissions near a black hole originate from a specific region whose size they can determine. They cannot, however, make a detailed map or picture of it. That’s the next stage — acquiring bona fide snapshots of a black hole and its environs — and it will take an expanded EHT network, with a broader global reach, to achieve it. Obtaining such images, Doeleman says, “will give us a much greater ability to tease out details. We’ll get to see in an unbiased way exactly what’s up there.”

The Big Picture Gets Bigger

In addition to witnessing the galactic light show as the G2 cloud falls into Sagittarius A*, the EHT could address more fundamental questions about our universe. More specifically, the researchers would like to capture the silhouette of a black hole — a bright ring of hot, luminous gas surrounding the dark, unrevealing interior of the black hole itself.

This glowing ring, called the last photon orbit, represents the innermost place that light can orbit a black hole without irrevocably falling in. It is just outside the event horizon. The ring’s shape, moreover, provides a testing ground for general relativity (Albert Einstein’s theory of gravity), which predicts a mostly circular shadow. “If we see a weird shape that deviates significantly from a circle, we can try to figure out what kind of deviation from general relativity is required to produce it,” says Avery Broderick, a theoretical astrophysicist at Waterloo University and the Perimeter Institute. To see the shadow, Broderick adds, you need an instrument as powerful as the EHT. “That’s the only thing around that can do it.”

Einstein unveiled his theory in 1915, and it has withstood every test experimentalists have thrown at it, so finding any departures from general relativity would be huge. Producing an image that puts this century-old theory of general relativity on trial won’t require any conceptual breakthroughs, says Doeleman. “It’s just a matter of putting more telescopes around the Earth” — installing more “light buckets” that can collect more photons and thereby amass more information while simultaneously boosting the angular resolution. He’s working on it.

The team’s near-term goal is to expand the EHT from its three present sites to nine or 10, including telescopes in Antarctica, Greenland, Mexico and the recently upgraded one at Haystack — “the dish that got me started,” as Doeleman notes. The biggest contribution to the system, by far, will come from the Atacama Large Millimeter Array (ALMA) in Chile, the largest radio telescope of its kind. That’s why Doeleman and his EHT colleagues are hard at work devising a way to extract radio signals from 50 or more radio dishes at ALMA without interfering with the array’s primary mission: studying the origins of the universe.

Going to a billion-dollar radio facility like ALMA, more than 16,000 feet above sea level, and getting it to mesh with the rest of their network is the kind of technological problem Doeleman relishes. Every component has to be double- and triple-checked, he says, to make sure the system as a whole can perform interferometry of the highest precision. “But for me, making a new telescope work in this way is really a rush.”

Of course, the project involves more than just feats of technical wizardry. Ultimately, it’s about capturing a glimpse of some of the most bizarre places in the universe — places we previously could only speculate about, he says. It’s about getting as close to a black hole as our technology will take us, pulling back from the edge and sharing the extraordinary view with the world.

Massive galaxy M87 with its black hole emissions.

X-ray and radio image by NASA/CXC/KIPAC/N.Werner and NSF/NRAO/AUIW. Cotton

Taking in the Universe’s Rare Sights

Doeleman and his EHT collaborators actually benefitted from a remarkable coincidence. It turned out, purely by chance, that the size of the EHT’s effective aperture meant that the smallest patch of space they could resolve closely matched the size of the event horizon of the Milky Way’s black hole, as currently understood.

In this case, everything lined up just right: The EHT is almost perfectly suited to its task. Doeleman chalks it up to the same good fortune that makes solar eclipses possible: “The moon has to be just the right size so that at a certain distance from Earth, it can completely blot out the sun. Nobody planned that. It just happened.”

And that’s not the only time the EHT team got lucky. The nearby giant galaxy M87 has a monster black hole at its center (more than 6 billion suns’ worth) offering astronomers a similar “eclipse effect,” notes Doeleman. “It’s about 2,000 times more massive than the Milky Way’s black hole, but it’s also about 2,000 times farther away, so the angular size is the same,” making it another ideal target for the EHT.

These two objects — which have the biggest apparent event horizons of any black holes that can be viewed from Earth — turn out to be complementary: Sagittarius A* is normally representative of quiet, inactive black holes, whereas M87 typifies the energetic black holes that produce elongated jets of gas and particles.

Those jets were of particular interest to Doeleman and company, who peered into M87 with the EHT in 2009 and again in 2012. The observations established that the region where M87’s radio emissions come from is not much bigger than the event horizon itself. By pinning down the source of the most intensely radiating material, he and his associates believe they have identified the base of M87’s powerful jets, which appear to emanate from just outside the black hole. “So we think we’re getting close to the edge of the black hole,” Doeleman says. These new insights on where the jets are formed should help theorists resolve another long-standing mystery regarding how the jets are formed.

[This article originally appeared in print as "To the Edge and Back."]